STRUCTURE AND DEVELOPMENT OF SPORANGIA 299 



granular thin- walled cells of the tapetum, which thus disorganises 

 before the sporangium matures. 



With the ripening of the sorus the indusium dries and shrivels, 

 exposing the sporangia, which likewise begin to lose moisture. 

 As evaporation proceeds, the thin outer walls of the annulus 



FIG. 1 66. Nephrodium filix-mas. A, Mature spores, greatly magnified. 



B, A single sporangium, showing the annulus (an.} and stomium (5.). 



C, Early stages (I-IV) in the development of the sporangium (after 

 Miiller). D and E, Successively later stages. a., archesporium ; 

 ap., apical cell ; g., gland ; St., cells of stalk ; t., tapetum ; w., wall. 

 F, Germinating spore (sp.}. p., beginning of prothallus ; r., rhizoid. 



commence to bulge inwards (cf. Figs. 165 and 166, B), owing 

 to the gradual reduction in the volume of their sap. Thus an 

 increasing tension is set up in the annulus, 1 which eventually 



1 It will be realised that the side-walls of the annulus are much less 

 thickened than the horizontal walls, so that the contraction is entirely in 

 the longitudinal direction. 



